There are three options (all called "scripts") for bringing subtitles into Encore, but (I think) they all get baked into the DVD structure as overlays that are basically images.
Text file scripts often have issues of font, formatting, etc. Image files avoid many of these.
Adobe Encore * Subtitle scripts
The issue I see for you is that you will have to downrez and recompress the mp4s for DVD. It is better to export the DVD files from the editing software or a lossless intermediate.